Thumbnail pictures for Kindle PW are stored at system\thumbnails directory. The thumbnail's file name must correspond to the book's file name. You may need to play around with the file name a bit to make it work.

@Julius thank you for your reply. Unfortunately, the problem does not lie in
the fact that the covers have a different name. Just the same file has a
cover uploaded when it is send by the cable and do not have it when I send
it by e-mail to @kindle.com.

I had hecked dozens of different factors (resolution, dimensions, seats,
etc.) and there is no difference.

@Julius thank you for your reply. Unfortunately, the problem does not lie in
the fact that the covers have a different name. Just the same file has a
cover uploaded when it is send by the cable and do not have it when I send
it by e-mail to @kindle.com.

I had hecked dozens of different factors (resolution, dimensions, seats,
etc.) and there is no difference.

eathan:

That's as may be, but books/documents sent through the PDS actually undergo conversion, just as though they were uploaded at KDP. Files sent through USB don't. That's why--without getting into all sorts of this and that--books loaded via USB will display the thumbs, and those emailed won't. Same thing happens, generally speaking on PDS-delivered mobis on the 1st-Gen Fire.